Preheat and prepare: Position a rack in the centre of your oven and preheat to 180°C (fan-assisted). Grease a 12-hole muffin tin with butter or line with paper cases. The tin should feel warmly hot when you hold your hand near it.

Whisk the dry ingredients: In a large bowl, sift together the flour, caster sugar, baking powder, bicarbonate of soda, salt, and 1 teaspoon of cinnamon. Whisk until they are evenly combined; the mixture will look pale and smell faintly sweet.

Combine the wet ingredients: In a separate jug, whisk together the buttermilk, whole milk, egg, melted butter, and vanilla extract. The mixture will be a smooth, pale yellow liquid with a slightly thick consistency from the buttermilk.

Mix gently: Pour the wet ingredients into the dry ingredients. Using a rubber spatula, fold together with as few strokes as possible. Stop as soon as you see no more streaks of flour—the batter will look lumpy and slightly shaggy. Over-mixing here will make the muffins tough.

Fill the tin: Divide the batter evenly among the 12 muffin cups, filling each about three-quarters full. The batter should be thick and mounded slightly in the centre. Tap the tin once on the counter to settle any air pockets.

Bake until golden: Bake for 12–14 minutes. The muffins will be domed, golden brown on top, and a skewer inserted into the centre should come out clean. The kitchen will smell like warm cinnamon and toasted butter.

Cool and coat: Let the muffins cool in the tin for 5 minutes. Meanwhile, mix the remaining 100g caster sugar and 2 tablespoons cinnamon in a shallow bowl. While the muffins are still warm (but not scorching hot), brush each one lightly with melted butter, then roll it in the cinnamon sugar until completely coated. The sugar will stick and create a crackly, glossy crust.